A known way of making a cable with high propagation velocity is to reduce the capacitance per unit length that exists between two conductors of the cable, by increasing the distance between the conductors and the distance between the conductors and a screen, and also by reducing the mean value of the permittivity of the dielectrics situated between the two conductors along the cable.
A known way of reducing the mean value of the permittivity of the dielectric between the two conductors of a cable consists in using an expanded plastic. For example, the permittivity of polyethylene is 2.28 when solid, and it is reduced to 1.6 or 1.5 when expanded. However, expanded dielectric materials suffer from the drawback of low mechanical strength which leads to the conductors moving relative to each other when the cable is subjected to bending or twisting. Such displacement gives rise to a local degradation in the electrical characteristics of the cable.
For example, European patent application No. 0 296 692 describes a cable for data transmission comprising:
an insulating core that is cylindrical in shape and that has four spiral-wound uniformly distributed longitudinal grooves formed therein:
four bare conductors placed in the grooves;
a first insulating layer surrounding the set of conductors and the core;
a metallic screen surrounding the insulating layer; and
a peripheral second insulating layer providing mechanical protection for the cable as a whole.
That cable is bulky since the diameter of the insulating core is much greater than the diameter of each of the four conductors. It maintains a predetermined distance between the conductors so as to reduce the capacitance per unit length between conductors. The core and the first insulating layer may be made of expanded polytetrafluoroethylene.
The object of the invention is to propose a cable having high propagation velocity without the drawbacks of known types of cable.